YOU SAID:

the first rule of purgatory is that you mustn't walk backwards

INTO JAPANESE

煉獄の最初の規則はあなたが後ろに歩いてはいけないということです

BACK INTO ENGLISH

The first rule of purgatory is that you should not walk behind

INTO JAPANESE

煉獄の最初のルールはあなたが後ろに歩かないでくださいということです

BACK INTO ENGLISH

The first rule of purgatory is that you should not walk behind